Juno Enters Jupiter's Magnetic Field

NASA's
Jupiter-bound Juno spacecraft has entered the planet's magnetosphere, where the
movement of particles in space is controlled by what's going on inside Jupiter.

"We've
just crossed the boundary into Jupiter's home turf," said Juno Principal
Investigator Scott Bolton of Southwest Research Institute, San Antonio.
"We're closing in fast on the planet itself and already gaining valuable
data."

Juno is
on course to swing into orbit around Jupiter on July 4. Science instruments on
board detected changes in the particles and fields around the spacecraft as it
passed from an environment dominated by the interplanetary solar wind into
Jupiter's magnetosphere. Data from Juno's Waves investigation, presented as
audio stream and color animation, indicate
the spacecraft's crossing of the bow shock just outside the magnetosphere on
June 24 and the transit into the lower density of the Jovian magnetosphere on
July 25.

"The
bow shock is analogous to a sonic boom," said William Kurth of the
University of Iowa in Iowa City, lead co-investigator for the Waves investigation.
"The solar wind blows past all the planets at a speed of about a million
miles per hour, and where it hits an obstacle, there's all this
turbulence."

The
obstacle is Jupiter's magnetosphere, which is the largest structure in the
solar system.

"If
Jupiter's magnetosphere glowed in visible light, it would be twice the size of
the full moon as seen from Earth," Kurth said. And that's the shorter
dimension of the teardrop-shaped structure; the dimension extending outward
behind Jupiter has a length about five times the distance between Earth and the
sun.

Out
in the solar wind a few days ago, Juno was speeding through an environment that
has about 16 particles per cubic inch (one per cubic centimeter). Once it
crossed into the magnetosphere, the density was about a hundredfold less. The density
is expected to climb again, inside the magnetosphere, as the spacecraft gets
closer to Jupiter itself. The motions of these particles traveling under the
control of Jupiter's magnetic field will be one type of evidence Juno examines
for clues about Jupiter's deep interior.

While
this transition from the solar wind into the magnetosphere was predicted to
occur at some point in time, the structure of the boundary between those two
regions proved to be unexpectedly complex, with different instruments reporting
unusual signatures both before and after the nominal crossing.

"This
unusual boundary structure will itself be the subject of scientific
investigation," said Barry Mauk of the Johns Hopkins University Applied
Physics Laboratory, Laurel, Maryland, who is the instrument lead for the
Jupiter Energetic-Particle Detector Instrument (JEDI) on Juno.